Prevention of refrigerant solidification

ABSTRACT

A refrigerant system may utilize CO 2  as a refrigerant. Should the sensed operating conditions indicate that the refrigerant might be approaching a condition at which the refrigerant could solidify, corrective actions are taken to prevent refrigerant transformation to a solid thermodynamic state. In one embodiment, hot gas from a compressor discharge is bypassed to a location upstream of the evaporator. In another embodiment, the high-side pressure of a refrigerant system is adjusted. In yet another embodiment, an additional charge of refrigerant is delivered on demand into the refrigerant system. In still another embodiment, a defrost cycle is initiated on demand. These embodiments prevent the refrigerant from approaching the conditions at which it may solidify.

BACKGROUND OF THE INVENTION

This application relates to refrigerant systems, which utilize CO₂ as arefrigerant, and which take preventive steps to reduce the likelihood ofthe CO₂ refrigerant transforming to a solid thermodynamic state.

Generally, refrigerant systems are utilized to circulate a refrigerantthroughout a refrigerant circuit to condition a secondary fluid to bedelivered to an indoor environment. As one example, air conditioningsystems circulate a refrigerant to condition air being delivered into aclimate-controlled space or zone.

Over recent years, a heightened concern about global warming, as wellas, in some cases, ozone depletion, caused by some of the most commonlyused refrigerants, such as R22, R123, R134a, R410A and R404A, forcedHVAC&R industry to search for alternative fluids and refrigerant systemsolutions. Therefore, much attention has been drawn to so-called naturalrefrigerants, such as R744 (CO₂), R718 (water) and R717 (ammonia). CO₂is one of these promising natural refrigerants that has zero ozonedepletion potential and extremely low (one) global warming potential.Thus, CO₂ is becoming more widely used as a replacement for theconventional HFC refrigerants. However, utilizing CO₂ as a refrigerantdoes raise challenges for the refrigerant system designer. One challengeraised by CO2 is that it can transform to a solid thermodynamic state atpressures which can be experienced in typical refrigerant systemapplications. The CO₂ refrigerant has a relatively high triple point. Asan example, with a pressure of about 75.1 psia, which would correspondto a saturated temperature of −69.8 degrees Fahrenheit, the CO₂refrigerant can solidify.

If the CO₂ refrigerant transforms to a solid thermodynamic state, therefrigerant system ceases to operate. The solid refrigerant could plugup the expansion device, the distributor and distributor tubes, theevaporator refrigerant heat exchange channels and associated refrigerantpipes. Among other undesired phenomena, it could also cause compressordamage. With the possibility that pressure in the refrigerant systemcould drop, on some occasions, below 75.1 psia, the potential for CO₂solidification raises challenges for the refrigerant system designer.Such situations can occur, for example, if the refrigerant system losessubstantial amount of charge, the expansion device has malfunctioned,the evaporator fan has ceased to operate properly, the evaporator heatexchanger got plugged, a low pressure sensor has malfunctioned, a lowpressure switch failed, etc. or a combination of thereof.

SUMMARY OF THE INVENTION

In disclosed embodiments of this invention, various preventive steps aretaken should the refrigerant system be approaching a situation wherein aCO₂ refrigerant can transition to a solid thermodynamic state. In oneembodiment, a bypass line selectively bypasses hot compressedrefrigerant gas upstream of an evaporator. This design concept willincrease pressure and temperature in the evaporator, preventing the CO₂refrigerant from transitioning to a solid thermodynamic state.

In another embodiment, in transcritical applications, the refrigerantsystem high-side pressure is reduced, should the conditions in theevaporator be approaching solidification conditions for the CO₂. Byreducing the pressure on the discharge (high-pressure) refrigerant side,the refrigerant distribution throughout the system is affected, causingthe evaporator pressure to change, preventing the solidification of theCO₂.

In another embodiment, a receiver may contain an additional CO₂refrigerant charge, which can be selectively delivered into therefrigerant system to increase the evaporator pressure, when therefrigerant system operation is approaching a situation where the CO₂refrigerant could solidify.

In yet another embodiment, should the conditions indicate that therefrigerant system is approaching a condition, which could causesolidification of the CO₂ refrigerant, a defrost operation at theevaporator is initiated, preventing the transformation of the CO₂refrigerant to a solid thermodynamic state.

In using those techniques, the refrigerant system can still continue tooperate without being shutdown, as would have been the case if therefrigerant system were stopped, for example, using a low-pressureswitch, which would trip the refrigerant system if the suction pressuredecreases below a certain specified pressure limit.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first schematic of the present invention.

FIG. 2 shows a second schematic of the present invention.

FIG. 3 shows a third schematic of the present invention.

FIG. 4 shows a fourth schematic of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a refrigerant system 20 having a compressor 22 delivering acompressed refrigerant through a heat exchanger 24. For operation abovethe critical point (supercritical operation), the heat exchanger 24 isnormally called a gas cooler and, for operation below the critical point(subcritical operation), the heat exchanger 24 is normally called acondenser. From the heat exchanger 24, the refrigerant is delivered toan expansion device 26, and then to an evaporator 28. As shown, apressure sensor 30 senses the evaporator pressure and transmits thesensed reading to a control 32. While the pressure sensor is shown atthe evaporator, other appropriate locations, such as a suction line or asuction port of the compressor 22, can be utilized, and parameters otherthan pressure, such as refrigerant saturation suction temperature, maybe sensed and deduced to the low-side refrigerant pressure.

In the present invention, should the control 32 sense that therefrigerant system could be approaching conditions at which the CO₂refrigerant could solidify, the hot gas bypass line 34 will deliver hotrefrigerant gas from the compressor discharge to a location upstream ofthe evaporator 28, by opening a refrigerant flow control device such asa valve 36. Potential locations for solidification include theevaporator 28 and the vicinity of the exit from the refrigerant systemexpansion device 26. In this manner, the low pressure conditions withinthe evaporator 28 will be avoided, and the CO₂ refrigerant will notsolidify.

FIG. 2 shows another embodiment 40, wherein when operating conditionsapproaching the transformation conditions of the CO₂ refrigerant into asolid thermodynamic sate are sensed by the sensor 30, the control 32reduces the high-side pressure for the refrigerant system 40, bycontrolling the opening of a variable (adjustable) orifice valve 232.When the opening of the valve 232 is strategically changed, therefrigerant is re-distributed throughout the refrigerant system suchthat the evaporator pressure is changed accordingly. Thus, thelikelihood of the CO₂ refrigerant solidification is reduced.

FIG. 3 shows another embodiment 50, wherein a receiver 52 containsadditional refrigerant charge to be selectively delivered into therefrigerant system 50. Should the conditions sensed by the sensor 30indicate that the refrigerant system is approaching a potentiallyproblematic situation causing CO₂ refrigerant solidification, a flowcontrol device such as a valve 54 is opened and additional refrigerantis delivered into the refrigerant system 50. In this manner, thepressure in the evaporator 28 is raised, and the solidification of theCO2 refrigerant is avoided.

FIG. 4 shows yet another embodiment 60, wherein a defrost cycle isinitiated by the control 32, should the conditions indicate the CO₂refrigerant is approaching a solidification line. In the illustratedembodiment, a defrost coil 62 associated with the evaporator 28 may beactuated to raise the temperature and pressure within the evaporator 28.

It should be pointed out that many different compressor types could beused in this invention. For example, scroll, screw, rotary, orreciprocating compressors can be employed.

The refrigerant systems that utilize this invention may have variousoptions and enhancement features, such as, for instance, tandemcomponents, economizer branches, reheat circuits, intercooler heatexchangers, etc., and can be used in many different applications,including, but not limited to, air conditioning systems, heat pumpsystems, marine container units, refrigeration truck-trailer units, andsupermarket refrigeration systems.

While preferred embodiments of this invention have been disclosed, aworker of ordinary skill in the art would recognize that certainmodifications would come within the scope of this invention. For thatreason the following claims should be studied to determine the truescope and content of this invention.

1. A refrigerant system comprising: a compressor for compressing arefrigerant and delivering it downstream to a heat rejection heatexchanger, refrigerant from said heat rejection heat exchanger passingthrough an expansion device and then through an evaporator, refrigerantfrom the evaporator returning to said compressor; and a control for saidsystem taking a corrective action, if said refrigerant system isapproaching a condition at which said refrigerant may solidify.
 2. Therefrigerant system as set forth in claim 1, wherein said control takesthe corrective action to prevent system shutdown.
 3. The refrigerantsystem as set forth in claim 1, wherein said refrigerant system ischarged with CO2 refrigerant.
 4. The refrigerant system as set forth inclaim 1, wherein said control takes the corrective action utilizing asensor for sensing a condition at which said refrigerant could solidify.5. The refrigerant system as set forth in claim 1, wherein saidrefrigerant can solidify in said evaporator.
 6. The refrigerant systemas set forth in claim 1, wherein said condition is a pressure of therefrigerant.
 7. The refrigerant system as set forth in claim 6, whereinsaid pressure is taken at a location associated with said evaporator. 8.The refrigerant system as set forth in claim 1, wherein said conditionis a temperature of the refrigerant.
 9. The refrigerant system as setforth in claim 8, wherein said temperature is taken at a locationassociated with said evaporator.
 10. The refrigerant system as set forthin claim 1, wherein a hot gas bypass line is positioned to take at leasta portion of refrigerant compressed by said compressor and deliver thisportion of refrigerant directly to said evaporator, and said controloperating a valve on said hot gas bypass line to expand this portion ofrefrigerant to a lower pressure, if said refrigerant system isapproaching a condition at which said refrigerant may solidify.
 11. Therefrigerant system as set forth in claim 10, wherein said portion ofrefrigerant is delivered to a location upstream of the evaporator. 12.The refrigerant system as set forth in claim 1, wherein said controlchanges a high-side pressure of the refrigerant system, if saidrefrigerant system is approaching a condition at which said refrigerantmay solidify.
 13. The refrigerant system as set forth in claim 12,wherein said high-side pressure is changed by controlling a valveopening.
 14. The refrigerant system as set forth in claim 1, whereinsaid refrigerant system further includes a receiver for storing anadditional charge of refrigerant, and a valve on a line communicatingsaid receiver into the refrigerant system, said control opening saidvalve to deliver additional refrigerant into the refrigerant system, ifsaid refrigerant system is approaching a condition at which saidrefrigerant may solidify.
 15. The refrigerant system as set forth inclaim 1, wherein a defrost cycle is associated with said evaporator, andsaid control actuating said defrost cycle, if said refrigerant system isapproaching a condition at which said refrigerant may solidify.
 16. Therefrigerant system as set forth in claim 15, wherein a defrost coil isassociated with said evaporator to provide said defrost cycle, andwherein said defrost coil being actuated by the control.
 17. A method ofoperating a refrigerant system comprising the steps of: providing acompressor for compressing a refrigerant and delivering it downstream toa heat rejection heat exchanger, refrigerant from said heat rejectionheat exchanger passing through an expansion device and then through anevaporator, refrigerant from the evaporator returning to saidcompressor; and a control for said system taking a corrective action, ifsaid refrigerant system is approaching a condition at which saidrefrigerant may solidify.
 18. The method as set forth in claim 17,wherein said control takes the corrective action to prevent systemshutdown.
 19. The method as set forth in claim 17, wherein saidrefrigerant system is charged with CO2 refrigerant.
 20. The method asset forth in claim 17, wherein said control takes the corrective actionutilizing a sensor for sensing a condition at which said refrigerantcould solidify. 21.-32. (canceled)